As semiconductor devices become more highly integrated, the design rules are reduced. Thus, in a dry etch process, it is important to have a high etch selectivity and a high anisotropy to properly etch a wafer. Also, it is important to be able to reproduce the same etch for each wafer that passes through the dry etch process.
In a dry etch process, the initial set of wafers that are processed can be spoiled when the dry etch processing chamber is not stabilized. In other words, if the ambient atmosphere within the chamber has not reached steady state, an unsteady etch rate can result.
This problem also occurs after a series of wafers have been processed or the dry etch process remains idle for a period of time between etch processes. Thus, a “seasoning process” is performed to prevent these problems by using a test wafer before a main etch process. Here, “seasoning process” means a process of etching a test wafer before performing a main etch process in the same chamber. In a conventional method, a recipe for a seasoning process is the same as that of a main etch process. However, immediately after finishing the seasoning process, the ambient atmosphere in the chamber may still not be stabilized, which results in an unsteady etch rate in the main etch process.
FIG. 1 is a graph showing experimental results obtained using a conventional dry etch process. In FIG. 1, the results of two test cases are shown. In case 1, a seasoning process was performed using a test wafer with an oxide layer in a dry etch chamber. Then, a main etch process was performed in the same chamber using a run wafer having a polysilicon layer and a tungsten silicide layer sequentially stacked on the wafer. Further, the run wafer included a hard mask pattern formed on the tungsten silicide layer. In the main etch process, the tungsten silicide layer was etched using the hard mask pattern by supplying Cl2 and SF6. An end-point detection time was determined as the time when the tungsten silicide layer was completely etched, as measured using an optical emission spectroscopy. Next, the polysilicon layer was etched by supplying HBr and O2. In a seasoning process (which was performed before the main etch process), a test wafer with an oxide layer was etched by supplying the same etch gases, under the same conditions and with the same sequences, as in the main etch process.
In case 2, the same main etch process was used as in case 1. However, in the seasoning process of case 2, a test wafer having a polysilicon layer was etched. Further, the main etch recipe used in case 2 was identical with that of case 1. In case 1 and case 2, each of the main etches were repeatedly performed on multiple wafers of at least one lot.
As illustrated in FIG. 1, with case 1, after the seasoning process was performed with respect to the test wafer with the oxide layer, an end-point detection time in the first main etch process was determined to be greater than the end-point detection times of subsequently processed wafers. As subsequent wafers were processed through the main etch process, the end-point detection time gradually decreased and stabilized. Furthermore, in case 2, after performing a seasoning process with respect to the test wafer with the polysilicon layer, an end-point detection time of a first main etch process was determined to be less than the end-point detection times of subsequently processed wafers. As subsequent wafers were processed through the main etch process, the end-point detection time gradually increased and stabilized. Thus, as is apparent from FIG. 1, although a seasoning process was performed in the conventional dry etch process with respect to a test wafer by using the same etch recipe with that of the main etch process, the conventional method did not provide a constant etch rate for the main etch process.
Therefore, a need exists for a reproducible dry etch process that reduces the number of wafers that are spoiled during the start-up of a dry etch process.